Regenerative type periodic flow devices are conventionally employed for the transfer of heat or of other constituents from one fluid stream to another, and thereby from one area or zone in space to another. Typically, a sorptive mass is used to collect heat or a particular mass component from one fluid stream which flows over or through the sorptive mass. The flowing fluid is rendered either cooler (in the case of heat sorption) or less concentrated (in the case of, for instance, adsorption of particular gases). The sorptive mass is then taken "off-stream" and regenerated by exposure to a second fluid stream which is capable of accepting the heat or material desorbed with favorable energetics.
In many instances, the sorptive material is contained within a vessel or distributed within a bed structure. It is desirable that such material be provided with maximum surface area, and that the fluid flow through the sorptive material matrix in a smooth (non-turbulent) and regular state. Once the sorptive material has been saturated (i.e., has reached its maximum designed capacity for sorption), the vessel or bed is then removed from the fluid flow path and exposed to a second fluid flow to regenerate the sorptive capacity of the material by, for instance, cooling the sorptive material or desorbing material taken up during "on-stream" operation. After such regeneration, the sorptive material is once more placed back "on-stream" and the operation continues.
From such single cycle systems evolved multiple vessel systems which permitted semi-continuous (or semi-batch) operation by synchronously alternating two or more sorptive vessels between on-stream and off-stream operation. The choice of numbers of vessels and cycle structures depends on many factors, but most importantly the ratio between consumption rate of the sorptive capacity of the vessel, and regeneration rates for that same vessel.
In some applications, semi-continuous systems have evolved into continuous flow systems where the sorptive media itself is moved between two or more flowing fluid streams. The most common construction employed for such systems is a porous disk, often referred to as a wheel or rotor. In its simplest form, such a wheel is divided into two flow zones, and fluid is passed over the sorptive surface of the wheel (typically flowing through the thickness of the disc parallel to the rotational axis of the cylinder) as the wheel is rotated to carry the sorptive material from one zone, into the other, and back again to complete a revolution. In a heat exchanger wheel, for instance, one zone of warm fluid and one zone of cooler fluid are present. Heat is adsorbed by the material of the wheel in the warm flow zone, and is carried away from the wheel as the sorptive material passes through the cool flow zone. U.S. Pat. No. 4,594,860 discloses such a continuous flow system and is hereby incorporated by reference.
FIG. 1 illustrates a schematic of a conventional open-cycle air-conditioning system, generally designated 9. A moisture transfer wheel assembly 11 constitutes the exterior or outside element of the system 9. As discussed hereinafter, the moisture transfer wheel 11 is separated into two sections to provide an intake path and an exhaust path through the moisture transfer wheel 11, as indicated by the arrows. A heat exchanger wheel assembly 13, also partitioned to provide intake and exhaust paths, is located substantially adjacent to the moisture transfer wheel 11, separated only by a heat regeneration coil 19. An auxiliary heating coil 21 may be placed in the system 9 for use in cold months when it is desirable to heat the interior of the area to be conditioned, rather than to cool it. The heat regeneration coil 19 and heating coil 21 include fluid pipes (not shown) which are interconnected with standard heating units (not shown), such as a solar heating unit. The system 9 terminates in a pair of evaporator elements 15 and 17 separated by a partition 6.
In a first type of system, generally referred to as a "make-up" system, the arrows 27 represent the intake air which is supplied to a building (not shown) having a closed and conditioned space (not shown) from an outside source, and the arrows 29 represent air exhausting from the enclosed and conditioned building space to an ambient space. A supply blower 23 and an exhaust blower 25 are provided to implement the necessary air movement within the system.
In a second known system, generally referred to as a "recirculation" system, the arrows 27 represent air which is drawn from within the enclosed and conditioned space, processed by dehumidification and/or cooling, and then returned to the enclosed space. The arrows 29 represent air from an outside or ambient source which is circulated through the regeneration side of the air conditioning system. Generally, recirculation systems add some air from the outside source to the recirculated air 27.
As is well known, the system 9 provides removal of the moisture from the intake air by the moisture transfer wheel 11. When moisture is removed from the air, the temperature of the air increases. The air is subsequently cooled upon passing through heat exchanger wheel 13, which lowers the temperature of the warm, dry air. Evaporator element 15 adds moisture to the air, thus reducing the temperature further and supplying cool air to the conditioned area. The exhaust air passes through evaporator element 17 and through heat exchanger wheel 13 to remove heat from the heat exchanger and raise the temperature of the exhaust air. The temperature of the exhaust air is further raised by means of the heat regeneration coil 19 to provide high temperature air in the exhaust path, resulting in regeneration of the moisture transfer wheel 11. The air from the moisture transfer wheel 11 is exhausted into the atmosphere. The system 9 is disclosed in U.S. Pat. No. 4,594,860, accordingly, further description of the structure of the system 9 is omitted for purposes of brevity only, and is not limiting.
There is a temperature relationship between the process or intake air leaving the moisture transfer wheel 11 and the required regeneration temperature of the regeneration or exhaust air entering the moisture transfer wheel 11. The temperature of the regeneration air entering the moisture transfer wheel 11 needs to be high enough to create a vapor pressure which is lower than the vapor pressure of the process air leaving the moisture transfer wheel 11, which is then moved to the heat transfer wheel 13. The regeneration air entering the moisture transfer wheel 11 also needs to have enough sensible energy for the condensed water trapped in the moisture transfer wheel 11 to vaporize and free itself from the moisture transfer wheel 11. In the past, the temperature of the regeneration air entering the moisture transfer wheel 11 was required to be a minimum of forty degrees Fahrenheit higher, and as much as one-hundred-and-fifty degrees Fahrenheit higher than the temperature of the process air leaving the moisture transfer wheel 11 toward the heat transfer wheel 13.
Where the moisture transfer wheel 11 can regenerate at a relatively low temperature, for instance, one-hundred-and-forty degrees Fahrenheit, the moisture transfer wheel 11 has the advantage of using waste heat from conventional air-conditioning and refrigeration condensers, among other sources. However, where the moisture transfer wheel 11 can regenerate at relatively low temperatures, there is a problem with treating process air which is drawn from ambient conditions which has the potential of being both high in temperature and humidity. Hot ambient temperatures limit the amount of moisture that can be removed from the incoming process air because the process air leaving the moisture transfer wheel 11 has to have a lower temperature than the regeneration temperature. As the process air passes through the moisture transfer wheel 11, the latent heat of vaporization is released by the water vapor being withdrawn from the air and, as a result, the air picks up the latent heat. The released latent heat increases the air temperature at a relationship of about 0.62 degrees-per-grain of moisture removed. When treating ambient air which is both high in temperature and humidity, less moisture can be absorbed by the moisture transfer wheel 11 and converted to sensible temperature before the process air begins to approach the regeneration temperature limit. Thus, the system will reach some equilibrium at a much-reduced latent capacity. Thus, a need has arisen for a regenerative-type air-conditioning system which can process ambient air which is both high in temperature and moisture without reducing the latent capacity of the system.
While the foregoing problem of not being able to treat process air which is derived from one-hundred-percent ambient air has been solved in the past by taking the process air from within the space to be conditioned (i.e., a recirculation unit), the use of a recirculation unit cannot meet the demands of all air-conditioning applications. For instance, where the space to be conditioned includes a supplemental relatively high volume exhaust system, such as in a commercial kitchen having large exhaust fans for removing air and smoke from the cooking area. In such an application, a recirculation unit would not meet the demand for replacing air within the enclosed space. As a result, a negative pressure would be created within the space to be conditioned thereby causing ambient air to enter the space to be conditioned whenever a door or window is opened.
An additional problem with both the make-up and recirculation systems is the reduced efficiency of the systems at start up. When the unit first starts up, there is less heat transferred from the process side by the heat transfer wheel because the moisture transfer wheel is not removing as much moisture from the process side (which gets turned into sensible heat). The process air leaves the moisture transfer wheel cooler, and in turn leaves the heat transfer wheel cooler which means less of a load for the process evaporator coil. The condenser is also operating at a lower temperature and pressure, and the regeneration air cannot pick up sufficient heat from the condenser coils to completely regenerate the moisture transfer wheel. Less heat of rejection is available from the compressor because its picking up less heat from the process evaporator cooler and the system therefore runs inefficiently with the compressor not being loaded to capacity at start up. As the system operates, it incrementally picks up additional heat and the system efficiency slowly improves.
The heat regeneration coil 19 and heating coil 21 of the system 9 are of conventional structure. That is, in conventional coil arrangements, the tubing is mechanically connected to fin sheets. The fin sheets are used to extend the surface area of the tubes to increase the coil's heat-transfer effectiveness. That is, a typical condenser or cooling coil is made up of finned tube sheets with good transfer conduction material, typically aluminum or copper, with holes punched in a pattern array through which the tubes are inserted. Through various means, the tubes are expanded to make good contact with the finned tube sheets. In a typical application, air passes over the tubes and fins and is either cooled or heated by the fluid flowing inside the tubes.
Since heat is conducted not only to the area that passes over the fin sheets, but from the warmest area on the finned surface to the coolest area on the finned surface, because of the high conduction of fin sheets, the fin sheets tend to average out the temperature across the coil, even though the fluid in the tube is at different temperatures between front and back. Thus, the upper and lower temperature limits to which the air can be heated or cooled is therefore limited, which directly affects the total heat transferred by the coil and the energy efficiency of the system 9. Thus, a need has arisen for a condensing or cooling coil which can minimize the effect of averaging out the temperature across the coil without losing the benefit of the use of fin sheets.
The temperature of the regeneration air leaving the moisture transfer wheel is also not uniform, but follows a gradient. The air leaving the relatively wet region of the moisture transfer wheel is cooler than the air leaving the relatively dry region of the moisture transfer wheel. Although the coils downstream of the moisture transfer wheel may tend to average out the temperature and ignore this gradient, additional system efficiencies could be gained if the system were designed to take advantage of these temperature differences.
The present invention solves the foregoing problems by using a recovery evaporator and separating the condenser coils into two sections. Efficiency at start up is improved by using the recovery evaporator coil to pick up more heat from the regeneration air prior to exhausting it to increase the heat of rejection of the compressor. Separating the condenser coils into two sections spaced by an air gap minimizes the averaging effect by preventing heat transfer across the fin sheets between the two sections. Locating the subcool condenser coil and the recovery evaporator coil in a stacked arrangement downstream of the moisture transfer wheel on the regeneration side allows the system to take advantage of the temperature gradient in the regeneration air leaving the moisture transfer wheel to increase the system efficiency. Additionally, for some of the systems the process air entering the moisture transfer wheel is pre-cooled to increase efficiency. By knowing the performance of the moisture transfer wheel at various dew points, the process air entering the moisture transfer wheel can be cooled to a specific dew point depending on the desired leaving-air humidity. Pre-cooling the process air entering the moisture transfer wheel increases the total moisture removed by the moisture transfer wheel 11 because the process air entering the moisture transfer wheel 11 could handle a greater increase in temperature (due to the latent heat of vaporization) before approaching the regeneration temperature limit.